The present invention relates to methods and apparatus for condensing a vapor component of a gas mixture and more particularly to the efficient condensation and recovery of solvent vapor from a gas mixture containing such vapor and inert gas.
During the curing of solvent borne resin coatings in a curing oven, the solvent is evaporated into the oven atmosphere. Traditionally, extremely high air flows were forced through such ovens to limit the solvent vapor partial pressure to values below the lower explosive limit of such vapors in air and to remove such vapors from the oven. These prior art curing processes are objectionable in that large air flows remove excessive amounts of heat from the ovens and consequently high levels of fuel consumption are required to continually supply such heat. In addition, the discharge of hydrocarbon based solvents such as acetone, toluene, etc. to the atmosphere degrade air quality. Although it is possible to incinerate solvents in such discharged air streams, additonal fuel is frequently required in order to sustain combustion and such solvents cannot be recovered for future use.
Various techniques have been proposed to recover solvent vapors from curing ovens or drying chambers such as the methods and apparatus illustrated in U.S. Pat. No. 4,150,494 which is assigned to the assignee of the present invention. In this patent, liquid nitrogen is utilized as a refrigerant for condensing solvent vapor withdrawn from a curing oven or the like with the resulting, vaporized nitrogen being returned to the oven to inert the same. This enables the recovery of condensed, liquid solvent and avoids the discharge of solvent vapor to atmosphere. The total gas flows through the curing oven are diminished and thus, the quantity of fuel necessary to maintain desired oven temperatures is also reduced. This reference also discloses a single pass heat exchange device for condensing solvent vapors against a countercurrent flow of liquid nitrogen and means for sensing the temperature of the condensed solvent so that the flow of liquid nitrogen to the heat exchange means can be controlled to avoid freezing of the solvent. However, the solvent vapor-inert gas mixture withdrawn from the oven atmosphere generally contains a minor component of moisture and it is frequently necessary to subject the solvent vapor to relatively low temperatures, below -32.degree. F. to condense acceptable fractions thereof. Moisture will freeze and tend to clog piping or other conduits utilized to recover condensed solvent when such low temperatures are employed. Although the degree of refrigeration supplied to condense solvent vapor may be reduced upon using lower flows of liquid nitrogen, a corresponding reduction in the solvent fraction actually condensed will result and consequently, the non-condensed gas leaving the heat exchange means will contain an unacceptably high partial pressure of solvent vapor. Typically, it is desired to remove by condensation at least 99% or more of the solvent vapor in the withdrawn oven atmosphere.
In order to facilitate the condensation of solvent vapor withdrawn from curing ovens in an inert gas mixture, it has been proposed to vaporize a cryogenic liquid to form a cold inert gas (i.e. nitrogen) and utilize the refrigeration in such gas to chill by indirect heat exchange the liquid phase of a solvent. The chilled solvent may then be utilized to condense solvent vapor supplied to a suitable heat exchange device as is illustrated in U.S. Pat No. 4,237,700 which is also assigned to the assignee of the present invention. However, this system requires relatively large flows of cold, inert gas through an endless conduit and, consequently, is not a particularly effective means for condensing solvent vapor as these systems are bulky and require considerable floor space in a curing plant or facility. A further proposal for utilizing the refrigeration of a cryogenic liquid to condense solvent vapors is described in French published patent application No. 2,349,113. This reference teaches an indirect heat exchange process wherein liquid nitrogen is utilized to condense vapors emitted from a drying chamber. The use of water cooled heat exchange means alone for the purpose of condensing a solvent vapor from a gas mixture is illustrated in U.S. Pat. No. 2,746,168. However, this system is limited in that intense refrigeration is not available from cooling water and frequently intense refrigeration is required in order to condense a high fraction of solvent vapor, typically 99% or more from a gas mixture. U.S. Pat. No. 4,053,990 describes another heat exchange system for condensing volatile vapors from a gas mixture although the use of cryogenic liquids is not contemplated by this reference.
Although the prior art techniques described above provide various approaches to the problem of condensing a vapor component from a gaseous mixture, none of these approaches is fully satisfactory for condensing virtually all of a solvent vapor from an inert gas mixture which may also contain minor components of moisture. Thus, the above noted prior art does not describe efficient processes or apparatus for condensing virtually all of the solvent vapor in inert gas withdrawn from an oven or drying chamber, etc. at a temperature of approximately 250.degree.-600.degree. F. in a safe, reliable and cost efficient method. Furthermore, prior art systems for recovering solvent have been limited in that variations in flow rates (or tying to multiple ovens) are not readily withstood by such systems. Consequently, for a solvent recovery system to be fully effective, it must be provided with a degree of tolerance toward variations in flows of gas mixtures supplied thereto and yet still efficiently and reliably condense a component from such mixtures. In addition, such systems must be able to condense different solvent vapors existing in various concentrations from inert gas mixtures thereof. Furthermore, condensation of solvent vapor components of a mixture containing moisture must be sustainable at low temperatures (e.g. -80.degree. F. or so) to fully condense such vapor yet avoid the freezing of moisture which will clog pipes or conduits and damage and even destroy pumping devices. The solvent condensed from a gaseous mixture should also be recovered at a temperature suitable for storage, i.e. not excessively hot (over 80.degree. F. or so) or excessively cold. Consequently, the prior art has exhibited a need for methods and apparatus for condensing the vapor component of a gas mixture, typically solvent vapor in inert gas with minor components of moisture in a manner which satisfies the foregoing requirements.